Conventionally, a three-way catalyst is disposed in an exhaust passage of an internal combustion engine in order to purify an exhaust gas discharged from the engine. As is well known, the three-way catalyst has an “oxygen storage function.” That is, when a gas flowing into the three-way catalyst (catalyst inflow gas) contains excessive oxygen, the three-way catalyst stores the oxygen and purifies the NOx. When the catalyst inflow gas contains excessive unburnt substance, the three-way catalyst releases the oxygen which has been stored to purify the unburnt substance. Hereinafter, the three-way catalyst may also simply be referred to as a “catalyst.”
A conventional air-fuel ratio control apparatus (conventional apparatus) comprises an upstream air-fuel ratio sensor and a downstream air-fuel ratio sensor that are disposed upstream and downstream of the catalyst in the exhaust passage, respectively. The conventional apparatus controls an “air-fuel ratio of a mixture supplied to the engine (air-fuel ratio of the engine)” so as to have an air-fuel ratio (detected upstream-side air-fuel ratio) represented by an output value of the upstream air-fuel ratio sensor become equal to a target air-fuel ratio (target upstream-side air-fuel ratio, target air-fuel ratio of the catalyst inflow gas). This control is also referred to as a “main feedback control.”
Further, the conventional apparatus calculates a sub feedback amount so as to have an output value of the downstream air-fuel ratio sensor become equal to a “target value corresponding to a stoichiometric air-fuel ratio”, and substantially changes the target upstream-side air-fuel ratio with the sub feedback amount to control the air-fuel ratio of the engine (refer to, for example, PTL 1). The air-fuel ratio control using the sub feedback amount is also referred to as a “sub feedback control.”